Paging receivers typically receive radio frequency signals which are decoded to recover data included therein. The data recovery process, however, is often subject to error due to fading of the signal or interference. Sources of interference may be, for example, other radio signals within the same geographic region or high frequency harmonics generated by the paging receiver itself. As a result, many techniques have been developed for correcting errors in signals received by paging receivers.
For radio signaling protocols, such as GSC (Golay Sequential Code) protocol or POCSAG (Post Office Code Standardization Advisory Group) protocol, in which error correction techniques are employed, error correction information is typically appended to each transmitted message. Thereafter, a paging receiver which receives the message utilizes the error correction information to verify and correct bits of the received message. The paging receiver, however, can only correct a predetermined number of bits included in each word of the message by utilizing the error correction information. Therefore, the error correction information is useless for words having more than the predetermined number of bits in error.
Some conventional paging systems, as a result, include paging terminals for transmitting messages more than a single time. One paging system, for example, transmits each message three times. Paging receivers included in the paging system commonly receive and decode all three of the messages. Subsequently, each word of a first message is corrected using the error correction information associated therewith. When a word included in the first message cannot be corrected, i.e., when more than a predetermined number of bits is in error, a corresponding word included in one of the other two messages is error-corrected and substituted for the word that cannot be corrected. In this manner, a paging receiver can conveniently form a complete, error-free message comprising, if necessary, words from all three of the repeatedly transmitted messages.
This method, however, does not always yield a complete message because, when the paging receiver is in a fading or noisy environment, it is entirely possible that a corresponding word included in all three messages will be uncorrectable. In this situation, the paging receiver either presents an erroneous message or does not announce reception of the message at all.
Thus, what is needed is a paging receiver which employs improved error correction techniques, thereby allowing for presentation of an error-free message even when words of the message include greater than a predetermined number of bits in error.